1. Field of the Invention
The present invention relates to a liquid crystal display for displaying pictures and the like, having an appropriate capacity, capable of ensuring the accurate operation of thin-film transistors and having a high aperture ratio.
2. Description of the Related Art
Because of advantages in weight reduction, device miniaturization and thickness reduction, liquid crystal displays have come into wide use. As is generally known, a twist nematic mode (TN mode) active matrix liquid crystal display, in particular, requires a relatively low driving voltage, dissipates power at a relatively low rate, and is capable of displaying pictures in high contrast and high quality.
A general TN mode liquid crystal display of this kind is formed by disposing two glass substrates each provided with a polarizer, a transparent electrode and an alignment layer opposite to each other with a space therebetween and with the respective directions of orientation of the alignment layers perpendicular to each other, and filling up the space between the two glass substrates with a twist nematic liquid crystal so that molecules of the twist nematic liquid crystal can be twisted through and angle of 90xc2x0.
Recently, the dependence of the visibility, i.e., viewing angle characteristic, of a general TN mode liquid crystal display of this kind on viewing angle has become a problem. FIG. 7 shows the viewing angle characteristic of a general TN mode liquid crystal display, in which shaded region corresponds to a viewing angle range in which contrast (CR) is 10 or above. As is obvious from FIG. 7, the visibility from lateral viewing directions of the TN mode liquid crystal display is satisfactory, the visibility from vertical viewing directions is not satisfactory, and the visibility from upper vertical viewing directions is very bad.
The applicant of the present patent application proposed a liquid crystal display element of a structure capable of solving such a problem in Japanese Patent Application No. 7-306276. In this previously proposed liquid crystal display element, spaced linear electrodes 12 and 13 of different polarities are formed only on the lower substrate 11 as shown in FIG. 8 and any electrode is not formed on an upper substrate 10 as shown in FIG. 9 instead of forming liquid crystal driving electrodes on both the upper substrate 10 and the lower substrate 11 disposed on the opposite sides of a liquid crystal layer, and a voltage is applied across the linear electrodes 12 and 13 to align liquid crystal molecules 36 in the direction of a lateral electric field created between the linear electrodes 12 and 13.
More specifically, the linear electrodes 12 are connected to a base line 14 to form a comblike electrode 16, the linear electrodes 13 are connected to a base line 15 to form a comblike electrode 17, the comblike electrodes 16 and 17 are disposed so that the linear electrodes 12 and 13 are arranged alternately at intervals, and the base lines 14 and 15 are connected to a power supply 18 and a switching device 19.
As shown in FIG. 10A, an upper alignment layer is formed on a surface of the upper substrate 10 contiguous with the liquid crystal, the upper alignment layer is treated so as to align the liquid crystal molecules 36 in the direction of the arrow xcex2, lower alignment layer is formed on a surface of the lower substrate contiguous with the liquid crystal so as to align the liquid crystal molecules in the direction of the arrow xcex3 parallel to the direction of the arrow xcex2, an upper polarizer film having a polarizing direction in the direction of the arrow xcex2 in FIG. 10A is laminated to the upper substrate 10, and a lower polarizer film having a polarizing direction parallel to the direction of the arrow xcex1 is laminated to the lower substrate 11. When any voltage is not applied across the linear electrodes 12 and 13, the liquid crystal display element remains dark. When a voltage is applied across the linear electrodes 12 and 13, the liquid crystal display element turns bright.
FIGS. 12 and 13 show a configuration of an actual active matrix liquid crystal driving circuit employing the structure of a liquid crystal display provided with the foregoing linear electrodes 12 and 13.
As shown in FIGS. 12 and 13, in which only a portion of the active matrix liquid crystal driving circuit corresponding to one pixel is shown, a gate electrode 21 and spaced, parallel, linear electrodes 22 are formed by patterning a conductive film on a transparent substrate 20, such as a glass substrate, a gate insulating layer 24 is formed over the gate electrode 21 and the linear electrodes 22, and a thin-film transistor T is formed by forming a semiconductor film 26 in an area on the gate insulating layer 24 corresponding to the gate electrode 21, and forming a source electrode 27 and a drain electrode 28 on the opposite sides of the semiconductor film 26, and a second linear electrode 29 is formed by processing a conductive film in an area on the gate insulating film 24 corresponding to the middle between the first linear electrodes 22.
As shown in plan view in FIG. 12, gate lines 30 and signal lines 31 are formed on the transparent substrate 20 to define rectangular pixel regions arranged in a matrix, the gate electrode 21, i.e., a portion of the gate line 30, is formed in a corner of the pixel region, the second linear electrode 29 is extended in parallel to the signal line 31 and is connected through a capacity electrode 33 to the drain electrode 28 overlying the gate electrode 21, and the first linear electrodes 22 are extended in parallel to and on the opposite sides of the second linear electrode 29.
Ends of the first linear electrodes 22 on the side of the gate line 30 are connected to a connecting line 34 extended in parallel to the gate line 30 in the pixel region, and the other ends of the first linear electrodes 22 are connected to a common electrode 35 extended in parallel to the gate line 30. The common electrode 35 is extended in parallel to the gate line 30 through a plurality of pixel regions to apply the same voltage to the linear electrodes 22 of all the pixel regions. One end portion of the second linear electrode 29 is extended to a position over the common electrode 35, a capacity electrode 36xe2x80x2 is formed at the end of the second linear electrode 29 so as to overlie a portion of the common electrode 35 in the pixel region, the capacity electrode 33 formed at the other end of the second linear electrode 29 overlies the connecting line 34. The capacity electrodes 33 and 36xe2x80x2 form capacitors together with the connecting line 34 and the common electrode 35 underlying and separated by the insulating layer 24 from the capacity electrodes 33 and 36xe2x80x2, respectively, to stabilize the operation of the thin-film transistor T when driving the liquid crystal.
Although this liquid crystal display of the foregoing configuration provided with a liquid crystal driving circuit is advantageous in its wide viewing angle, the same has a problem that its aperture ratio is liable to be small.
The capacitor consisting of the capacity electrode 33 and the connecting line 34 formed on the opposite sides of the insulating layer 24 and the capacitor consisting of the capacity electrode 36xe2x80x2 and the common electrode 35 formed on the opposite sides of the insulating film 24 shown in FIGS. 12 and 13 need to have capacities on an appropriate level to stabilize the operation for driving the thin-film transistor T. Therefore, the common electrode 33, the connecting electrode 34, the common electrode 35 and the capacity electrode 36xe2x80x2 must be formed in widths as shown in FIG. 14 greater than those shown in FIG. 12. If the common electrode 33, the connecting line 34, the common electrode 35 and the capacity electrode 36xe2x80x2 are thus formed as shown in FIG. 14, the sum of the respective areas of the common electrode 33, the connecting line 34, the common electrode 35 and the capacity electrode 36xe2x80x2 increases relative to the area of the pixel region and, consequently, the aperture ratio of the liquid crystal display is liable to be reduced.
A disadvantage attributable to a small aperture ratio may be compensated by adjusting the intensity of the backlight backlighting the liquid crystal display element of the liquid crystal display, which, however, increases the power consumption of the liquid crystal display. Therefore, such measures to compensate the disadvantage have an adverse effect on the development of a liquid crystal display capable of reducing power dissipation.
The foregoing active matrix liquid crystal display drives molecules of the liquid crystal by creating a lateral electric field and hence needs a liquid crystal driving voltage higher than that needed by a TN mode liquid crystal display, which also increases power consumption.
Accordingly, it is an object of the present invention to provide a liquid crystal display capable of achieving both the realization of a high viewing angle characteristic by a configuration for driving a liquid crystal by a lateral electric field parallel to the surface of a substrate and the stable driving of thin-film transistors, and having a large aperture ratio.
Another object of the present invention is to provide an active matrix liquid crystal display of a lateral electric field drive system having a large aperture ratio and capable of suppressing power dissipation.
According to a first aspect of the present invention, a liquid crystal display comprises a pair of substrates disposed with a space therebetween, a liquid crystal filling up the space between the pair of substrates, a plurality of pixel electrodes formed in a plurality of pixel regions on an inner surface of one of the pair of substrates, common electrodes each for creating an electric field of a direction parallel to the inner surface of the substrate in cooperation with each of the plurality of pixel electrodes, and capacitor forming electrodes each formed over and spaced from the pixel electrode so as to form a capacitor in combination with each of the pixel electrodes.
In this liquid crystal display, each of the pixel electrodes may be formed in an inner portion of each of the plurality of pixel regions, the common electrode defining each of the pixel regions may be formed opposite to the pixel electrode, and the capacitor forming electrode may be formed in an inner portion of the common electrode.
Since an electric field of a direction parallel to the surface of the substrate can be created by the common electrode and the pixel electrode formed on the substrate, the alignment of molecules of the liquid crystal can be controlled by creating and removing the electric field to select a displaying mode or a nondisplaying mode. Since the common electrode is provided with a capacitor forming electrode, the common electrode is able to form a capacitor in combination with the pixel electrode.
In this liquid crystal display, corresponding portions of the pixel electrode and the capacitor forming electrode may be formed. in the shape of a strip, and the width of the portion of the pixel electrode may be greater than that of the corresponding portion of the capacitor forming electrode.
Since the width of the portion of the pixel electrode may be greater than that of the corresponding portion of the capacitor forming electrode, the capacitor forming electrode is covered with the pixel electrode. Accordingly, an aperture ratio does not fall and the liquid crystal is unaffected by providing with the capacitor forming electrode.
In this liquid crystal display, the common electrodes may be formed on the substrate provided with the pixel electrodes, and the pixel electrodes may overlie the common electrodes, respectively.
Thus, the pixel electrodes can be disposed near the liquid crystal and a high effective voltage is available for liquid crystal driving.
According to a second aspect of the present invention, an active matrix liquid crystal display of a lateral electric field drive system comprises: a pair of transparent substrates disposed with a space therebetween; a liquid crystal filling up the space between the pair of substrates; gate lines formed so as to extend in one direction on an inner surface of one of the pair of substrates; signal lines formed on the inner surface of the same substrate so as to extend across the gate lines and to define a matrix of pixel regions serving as pixels together with the gate lines; pixel switching elements connected to the pixels and the gate lines, respectively, each pixel switching element being driven by a gate voltage applied thereto through the gate line to apply a signal voltage through the signal line to the pixel electrode; pixel electrodes formed in the pixel regions and connected to the pixel switching elements, respectively; and common lines extended in parallel to the gate lines and having common electrodes extending toward the pixel electrodes, respectively, a common voltage being applied to each common electrode to create an electric field substantially parallel to the surface of the transparent substrate in cooperation with the pixel electrode to drive molecules of the liquid crystal to display pictures.
In this active matrix liquid crystal display, the pixel electrode corresponding to each pixel has an extension extended over the gate line corresponding to the adjacent pixel, capacitors for storing the signal voltage to be applied to each pixel are formed by overlapping portions of the pixel electrode and the common electrode, and overlapping portions of the extension of the pixel electrode and the gate line corresponding to the adjacent pixel.
In this active matrix liquid crystal display, a plurality of common electrodes may be formed for each pixel, the free ends of the plurality of common electrodes may be interconnected by a connecting electrode to form the capacitor for storing the signal voltage to be applied to the pixel by overlapping portions of the connecting electrode and the pixel electrode.
In this active matrix liquid crystal display, the common voltage applied to each pixel may be varied in synchronism with the application of the gate voltage to the switching element of the same pixel.
Since the pixel electrode and the gate line of the adjacent pixel form part of the capacitor for storing the signal voltage, a necessary storage capacity can be secured even if the area of the opposite portions of the pixel electrode and the common electrode is small. Consequently, the active matrix liquid crystal display has a high aperture ratio. Although the conventional active matrix liquid crystal of a lateral electric field drive system having a low aperture ratio compensates the disadvantage attributable to the low aperture ratio by increasing the intensity of the backlight, active matrix liquid crystal display of the present invention does not need any power to enhance the lighting effect of the backlight because the same has a high aperture ratio.
An expected electric field can be created between the pixel electrode of the pixel and the common electrode without increasing the amplitude of the signal voltage greatly. Accordingly, the power consumption of a source driver which generates the signal voltage can be suppressed, and the source driver may be such as having a low withstand voltage.